Project: Evolution of interactions between Wolbachia and its hosts: Drosophila model systems. Project Summary: Wolbachia are bacteria that live inside cells of their invertebrate hosts. They are generally maternally transmitted and often spread through populations by manipulating host reproduction. Their most commonly documented reproductive manipulation is cytoplasmic incompatibility (CI), increased embryo mortality when infected males mate with uninfected females. CI drives Wolbachia into populations and is being used to introduce into natural mosquito populations Wolbachia strains that suppress disease-causing viruses (particularly dengue fever). With their ability to block pathogen transmission, Wolbachia hold significant promise for controlling many diseases transmitted between humans by insects, including malaria and West Nile virus. Because Wolbachia are maternally inherited, they evolve to help their hosts survive and reproduce, for instance, by suppressing pathogens and increasing host fecundity. Conversely, because Wolbachia reproductive manipulations kill embryos, hosts may evolve to suppress these deleterious effects (without compromising beneficial effects). Health applications of Wolbachia depend on understanding their spread in nature and the potential for rapid Wolbachia-host coevolution. Knowledge of Wolbachia-host interactions in nature is limited to a handful of model systems. Wolbachia infections of Drosophila provide paradigms for understanding rapid spatial spread of Wolbachia, coevolutionary change, and the molecular mechanisms underlying these phenomena. Two of the best understood Wolbachia infections are those in D. simulans (especially wRi) and D. melanogaster (wMel). These infections persist in nature by fundamentally different mechanisms, with only wRi-simulans fully explained. Yet, the wMel-melanogaster association is apparently much older. This project aims to develop a deep understanding of Wolbachia population dynamics and evolution by: (1) expanding field and laboratory analyses of wRi-simulans and wMel-melanogaster, and (2) describing Wolbachia-host interactions and coevolution using at least 30 additional Drosophila species with Wolbachia infections. Tools from genomics, cell biology and evolutionary genetics, already optimized for Drosophila, will be used to study population and evolutionary dynamics in nature. For instance, microinjection techniques will move Wolbachia between Drosophila species and disentangle Wolbachia from host effects. Over the past 20 years, wRi and wMel have spread in Australia, and wRi has evolved in California. These current events provide a unique opportunity to follow population and evolutionary dynamics in action. Over 30 additional Drosophila species, known to carry Wolbachia, will be studied in detail to understand more generally the trajectory, time-scale and mechanisms of coevolution between Wolbachia and their hosts. The proposed research, based on a combination of field surveys, field and laboratory experiments, comparative genomics and lab assays of phenotypic effects, will provide the foundation for understanding likely Wolbachia trajectories in other systems, including applications to mosquito vectors of disease.